134 THE ATMOSPHERE AND ITS SERVICE TO MAN 



and increases the temperature of its surroundings; when 

 it expands it absorbs heat and lowers the temperature of its 

 surroundings. 



The greater the pressure on a liquid surface, the higher 

 is the boiling point ; the lower the pressure, the lower the 

 boiling point. This principle, along with the principle that 

 a substance absorbs heat as it changes from a liquid to a 

 gaseous state, underlies the operation of cold storage and 

 ice-manufacturing plants. 



The barometer is an instrument for measuring atmos- 

 pheric pressure. Since atmospheric pressure decreases with 

 altitude, a barometer may be used to measure altitude. 



QUESTIONS 



What are the characteristics and principal uses of the three most 

 abundant gases in the atmosphere ? 



What experiences have you ever had which show that hot air 

 will hold more moisture than cold air? 



How have you ever seen cooling by evaporation used ? 



In what ways does the moisture in the atmosphere affect bodily 

 comfort ? 



How can it be shown that the air has weight and exerts pressure ? 



What effect has heat upon the weight and volume of the atmos- 

 phere ? 



Suggest several methods for properly ventilating a house. 



What effect has pressure upon the weight and volume of air? 



Explain the construction of three machines which make use of 

 atmospheric pressure. 



In what way do compression and expansion affect the tempera- 

 ture of a gas ? 



How are the boiling points of liquids affected by pressure? 

 What practical uses are made of this principle ? 



How is ice manufactured? 



How do the two kinds of barometers ordinarily used differ in 

 construction ? 



